PRIOR CONCURRENT THERAPY: Biologic therapy: No prior transplantation or stem cell therapy No concurrent interferon therapy during initial melphalan and prednisone regimen Chemotherapy: See Disease Characteristics Concurrent other chemotherapy allowed if disease progression on study therapy Endocrine therapy: Not specified Radiotherapy: No more than 6 months since prior localized radiotherapy Concurrent localized radiotherapy allowed Surgery: Not specified Other: At least 30 days since prior investigational drug therapy Concurrent bisphosphonates for bone disease required No other concurrent enzyme preparation (including over the counter or nutraceutical preparations) No concurrent participation in other clinical study No concurrent anticoagulant therapy unless medically indicated